Such fill-level measuring methods are applied in a large number of branches of industry, e.g. in the processing industry, in the chemicals industry or in the foods industry.
Typically, the fill-level measuring device is mounted above the container. In measurement operation, microwave signals are transmitted from the fill-level measuring device toward the fill substance in the container, and their fractions reflected back in the container to the fill-level measuring device are received back as received signals after a travel time dependent on their path distance traveled in the container. Based on the received signals, echo functions are derived, which show amplitudes of the received signals as a function of a position corresponding to their travel time or their path distance traveled in the container. In such case, travel time and path distance are convertable into one another based on the propagation velocity of the microwave signals along the path.
Reflections on reflectors located in the container, reflectors such as e.g. the surface of the fill substance and the container floor, bring about local maxima in the echo functions, referred to herein subsequently as echoes, at a position in the echo function corresponding to their distances from the fill-level measuring device.
For determining the travel times, all known methods can be applied, which enable relatively short distances to be measured by means of reflected microwaves. The most well known examples are the pulse radar method and the frequency modulation, continuous wave radar method (FMCW radar).
In the case of pulse radar, short microwave transmission pulses are transmitted periodically into the container, where they are reflected and, after a travel time dependent on the path distance traveled by them, received back.
In the case of the FMCW-method, a microwave signal is continuously transmitted, which is periodically linearly frequency modulated, for example, in the manner of a sawtooth function. The frequency of the received signal consequently has, relative to the instantaneous frequency, which the transmission signal has at the point in time of receipt, a frequency difference, which depends on the travel time of the associated microwave signal. The frequency difference between transmission signal and received signal, which can be won by mixing both signals and evaluating the Fourier spectrum of the mixed signal, corresponds, thus, to the travel time and therewith to the separation of the reflecting surface from the fill-level measuring device. Furthermore, the amplitudes of the spectral lines of the frequency spectrum won from the Fourier transformation correspond to the echo amplitudes. This Fourier spectrum consequently represents the echo function in this case.
Based on the echo function, a fill-level echo is determined, which corresponds to the reflection of the transmitted signal on the surface of the fill substance. From the travel time of the fill-level echo, there results directly, in the case of known propagation velocity of the microwaves, the path distance, which the microwaves traveled on their path from the measuring device to the surface of the fill substance and back. Based on the installed height of the fill-level measuring device over the container, the sought fill level can be directly calculated therefrom,
In such case, it is desired also to be able to detect the presence of an empty container correctly and reliably. In the case of empty container, there is no reflection of the microwave signals on the surface of the fill substance. Instead, the microwave signals strike, at a distance from the fill-level measuring device subsequently referenced herein as the empty distance, a portion of the container floor, and are reflected there. In measurement applications, in which, in the case of empty container, at least a part of microwave signals transmitted into the container is reflected back via a container floor to the fill-level measuring device, the echo functions have in the case of empty container regularly an echo designated subsequently herein as the container floor echo, which is attributable to the reflection on the container floor.
Problematic, in such case, is that the position of the container floor echo in the echo function in the case of empty container is dependent on the location of installation of the fill-level measuring device and on the geometry of the container. Only in the case of empty container with flat floor are the transmitted microwave signals reflected directly back to the fill-level measuring device. In that case, the container floor echo occurs in the case of empty container at the position within the echo function corresponding to the empty distance.
If, in contrast, the transmitted microwave signals fall in the case of empty container, for example, on an inclined or curved portion of the container floor, then they are reflected corresponding to their angle of incidence, with which they strike the portion, and can, thus, only indirectly be reflected back to the fill-level measuring device via other reflections in the container. Here, in the case of empty container, the fractions of the transmission signals reflected back in the container to the fill-level measuring device travel a path dependent on the location of installation and dependent on the container geometry. The length of this path can, in given cases, be very much greater than the empty distance.
In order to detect an empty container, presently there are already methods applied, in the case of which, based on the echo functions, it is checked, first of all, whether a fill-level echo can be identified within the empty distance. If such is not the case, then, it is checked whether a container floor echo has occurred in a region of the echo functions exceeding the empty distance. Since the container floor echo, depending on container form, can also occur at positions, which are clearly greater than the empty distance, there is, for this, regularly taken into consideration a relatively large range of distances, which range begins at the position corresponding to the empty distance and contains those positions corresponding to distances to the fill-level measuring device, which are greater than the empty distance. If there is in this range of distances a corresponding echo, it is recognized as the container floor echo, and the fill-level measuring device reports an empty container.
This method contains, however, the danger of incorrect empty reports, when a fill-level echo of an actually present fill substance is temporarily not detectable. In that case, due to a container floor echo present in the echo function supplementally to the not detectable fill-level echo, an empty container is indicated.
This case arises, for example, when the fill substance has temporarily a smaller dielectric constant, and, thus, only weak reflections occur on the surface of the fill substance. These weak reflections are not unequivocally identifiable in the echo function due to their small amplitude. Likewise, also turbulence on the surface of the fill substance or foam formation on the surface can lead to the result that the fill-level echo can no longer be identified in the echo function.
Moreover, the fill-level echo can also no longer be directly identified, when the fill level in the container runs over a height, in which a microwave reflecting disturbance, such as e.g. a component installed in a container, such as a stirrer, is located. If the fill level is located at the height of the disturbance, then disturbance echo and fill-level echo superimpose in the same region of the echo function and, accordingly, one cannot be directly distinguished from the another. This problem is, however, today overcomeable by supplemental measures, such as e.g. suitable echo tracking algorithms.